1. Field of the Invention
The present invention relates to an image forming apparatus for providing a uniform image and a method for controlling a multi-phase stepping motor for use in an image forming apparatus.
2. Description of Related Art
In a facsimile apparatus or the like, image data received from another facsimile apparatus of the other party through a communication channel is sequentially stored in a recording buffer provided within a RAM (random-access memory), and then outputted through a recording apparatus to a recording paper. When the received image data is recorded on the recording paper, line feed control patterns corresponding to the amount of received image data stored in the recording buffer are set in advance with respect to various parameters. Then, the line feed operation for the recording paper is controlled in accordance with the line feed control patterns.
The line feed control which uses the density of recording as one parameter, for example, is shown in FIG. 5. In this control, the line feed interval (LF interval) is divided into 8 different stages. The LF interval is a length by which the recording paper is advanced at every constant time in a line feed fashion. The LF interval is progressively shortened from LF1 to LF8. When the amount of received image data stored in the recording buffer is large, the LF interval is lengthened to speed up the recording speed. Conversely, when the amount of received image data is small, the LF interval is shortened to slow down the recording speed. Thus, the LF interval is switched in response to the amount of received image data stored in the recording buffer.
However, if the LF interval is switched successively, the density of an image (recording image density) recorded actually in a portion in which the LF interval is long is decreased while the recording image density of an image recorded actually in a portion in which the LF interval is short is increased, even when the density set by the amount of ink, toner or the like to be supplied is made constant to record the image on the recording paper with a predetermined density. As a result, the image cannot be recorded on the recording paper with a constant recording image density, resulting in irregular or non-uniform image density on the recording paper.
Therefore, to record an image on a recording paper with a predetermined recording density, the set density is changed in response to the change of the LF interval with respect to each density parameter, i.e., line feed control pattern is set in advance. That is, when the LF interval is long, the set density is increased. When on the other hand the LF interval is short, the set density is decreased. Thus, an image is recorded with respective density parameters, there can be provided respective constant recording image densities.
In order to advance the recording paper in a line feed fashion, a 4-phase stepping motor 10 of a 2-phase exciting system is used, for example.
As shown in FIG. 6, this stepping motor 10 comprises a stator comprising a first winding unit 11 having two exciting phases (A-phase and C-phase) and a second winding unit 12 having two exciting phases (B-phase and D-phase) and a rotor 13 having alternately-arranged two magnetic polarities (not shown). Magnetic poles for A-phase and C-phase are alternately disposed on the inner wall of the first winding unit 11 at an equal interval in the circumferential direction. Also, magnetic poles for B-phase and D-phase are alternately disposed on the inner wall of the second winding unit 12 at an equal interval in the circumferential direction.
The first winding unit 11 and the second winding unit 12 are assembled in such a manner that the magnetic pole for the A-phase and the magnetic pole for the C-phase of the first winding unit 11 are arrayed on the axial symmetries (dotted lines) of the magnetic pole for the B-phase and the magnetic pole for the D-phase of the second winding unit 12. As a result, the magnetic poles are arrayed on the inner periphery of the winding unit at an equal interval along the circumferential direction in the sequential order of A, B, C, D.
However, in actual practice, the magnetic poles A, B, C, D are likely to be not arrayed at an equal angular interval accurately due to a positioning error caused when the first winding unit 11 and the second winding unit 12 are assembled.
Thus, when the 2-phase exciting control is carried out, the rotation angle of the rotor 13 provided when the A-phase and B-phase are excited and the rotation angle of the rotor provided when the B-phase and the C-phase are excited differ from each other. This causes a problem that the feed amount of the recording paper advanced by the stepping motor 10 in a line feed fashion is not uniform. That is, even though a recording pattern with respect to the excitation from a specified exciting phase is set, when an excitation from other exciting phase is carried out, a similar recording pattern cannot be provided.
For example, when there is set a line feed control pattern which is set based on a relationship between an LF interval and a predetermined density parameter in response to the case in which the magnetic poles are excited from the A-B phases upon start of the recording, if the magnetic poles are excited from the B-C phases to start the recording, then the LF interval does not correspond to that provided in which the magnetic poles are excited from the B-C phases. Therefore, even when it is intended to record image data with the predetermined density parameter, a density of recorded image does not become uniform. Furthermore, because the feed amount of the recording paper is different, a recorded image also is changed.
As a result, it is disadvantageous that, when recording is performed on the recording paper based on the received image data, a resultant image becomes different due to a difference of exciting phases (recording start exciting phases) of the magnetic poles excited at the beginning of the recording operation.
In this case, in order to provide a uniform image on a recording paper, it is necessary to set the specific line feed control pattern for each recording start exciting phase with respect to a predetermined parameter so that the line feed control pattern is switched in correspondence with the recording start exciting phase from which the recording is started.